The present invention relates to an optic system of illumination for a videoprojector based on DMD (Digital Micromirror Device) technology.
Videoprojection systems based on DMD (Digital Micromirror Device) technology are increasingly spreading, above all for the excellent image quality they are able to obtain, in particular for the brightness and resolution of the image itself, and also for their smaller projectors sizes compared e.g. to the devices utilizing kinescopes. A DMD device consists essentially of a set of aluminum square mirrors, with a side of micrometric size, e.g. 16 xcexcm, each one associated to an element of the image to be projected. i.e. to a pixel. Said mirrors can have a small angle rotation around a diagonal, such as xc2x110 degrees, where rotation in either direction is produced by two electrodes located under the mirror in opposite positions with respect to the rotating axis. Therefore, the light hits the mirror with an angle of about 20 degrees with respect to the perpendicular to the mirror plane when the latter is in its xe2x80x9crestxe2x80x9d condition, i.e. not attracted by any of the two electrodes. If the mirror is rotated in one direction, the reflected ray undergoes a deflection that does not affect in the projection lens and therefore, it is not sent to the screen. Therefore, the corresponding pixel is xe2x80x9coffxe2x80x9d. If rotation occurs in the opposite direction the pixel is xe2x80x9conxe2x80x9d, since the reflected light affects on the projection lens and is sent to the screen.
To each pixel of the image is associated a cell of a static memory of the type SRAM (Static Random Access Memory), containing the information for directing the electrodes that cause mirror rotation. Even if the reflected light always has the same intensity, changing the time during which a pixel remains xe2x80x9conxe2x80x9d, is obtained the effect of a luminosity change due to the integrating action produced by the human eye. A videoprojector may comprise only one DMD device, in which case its mirrors are illuminated sequentially by the three primary colours, i.e. red, green and blue, which are obtained sending the light of the lighting lamp to a revolving wheel, called colour wheel, divided in at least 3 segments, each one consisting of a dichroic filter, i.e. selective with respect to the wavelength, related to one of the 3 primary colours. Wheel rotation causes the light beam sent to the DMD device to take all three different colours sequentially. In the event, vice-versa, of a videoprojector with three DMD devices, the light of the lighting lamp is split in the three colours by a prism, and each colour is sent to a different DMD device.
In a DMD videoprojector, the choice of the optic system of illumination has taken on particular importance, since both the dimensions and utilization procedures of the videoprojector itself depend on it.
A first known illumination system is illustrated in FIG. 1 by means of a basic diagram. This basic diagram is plotted assuming that a videoprojector 21 is placed in horizontal position for frontal projection to a vertical screen 22, and therefore, said diagram corresponds to a plan view of said videoprojector 21: this premise applies to all subsequent figures, unless otherwise specified. Moreover, for clarity""s sake, the blocks indicated with the same reference number in the various figures have the same function. With reference number 1 is indicates a lighting lamp with a parabolic reflector, number 2 indicates an aspheric condenser focusing the light at the input of an integrating rod 4 consisting of an optic glass parallelepiped, whose function is to obtain a uniform light beam from the lighting lamp 1. The integrating rod 4 is preceded by a colour wheel 3, which, as said above, allows the reproduction of the colours through its dichroic filters in those videoprojectors using only one DMD device, as in the example of FIG. 1. In some instances, the distance from the lighting lamp 1 to the colour wheel 3 is closed by a collector, not shown in the figure, whose purpose is to hinder that reflected rays are spread in the surrounding space illuminating the environment The output light from the integrating rod 4 is collected by a lens system, in the specific instance three converging lenses, known as relay lens, and indicated collectively with the reference number 5. Said lenses 5, along with a mirror 6 and a prism 7 convey the light emitted by the lighting lamp 1 towards an image microforming device. i.e. a DMD device, indicated with number 9, on which is formed an focused image which is enlarged with respect to the one at the output of the integrating rod 4. This illumination diagram, in which the focusing occurs on the image microforming device 9, is known as a critical or Abbe""s illumination. The optical path from the lighting lamp and the image microforming device 9 undergoes two deflections: a first deflection due to the reflecting surface of the mirror 6; and a second deflection due to the prism 7. Said prism 7 conveys the light beam towards the image microforming device 9 with an angle of about 20 degrees, as requested by the manufacturer""s specifications for the image microforming device 9. The prism 7 is a common prism, such as TIR (Total Internal Reflection), i.e. operating with full reflection, for the presence of an air layer of about 10 xcexcm separating it from a second prism indicated with reference number 8. Said prism 8 deflects the light beam coming from the micromirrors on the surface of the image microforming device 9 towards a projection lens indicated with the reference number 10, which projects the image on a vertical screen 22.
A dotted line in FIG. 1 also indicates the optical path of the light beam emitted by the lighting lamp 1. A first segment AB, directed along the illumination axis of the lighting lamp 1, departs from a point A in line with said illumination lamp 1 to reach a point B in line with the mirror surface 6. Said first segment AB lies in a first plane indicated with P1 in FIG. 1a, where a basic perspective view of the optical path is reported within the videoprojector 21.
Then the light beam is deflected upwards by the mirror 6, as it can be clearly noticed in FIG. 1a, and reaches a point C pertaining to a second plane P2 located on the prism 7, wherefrom it is reflected to a point D pertaining to the surface of the image microforming device 9. As mentioned above, the image is formed by the image microforming device 9 modulating the light beam. Finally, said modulated light beam reaches a point E directly outside of the projection lens 10, i.e. identifying a projection segment DE, which is part of the projection axis. The extension of the segment DE reaches the screen 22.
It should be noticed that the mirror 6 deflects the optical path upwards along the segment BC, i.e. it is inclined, appearing in the plan view like a rectangle instead of a segment. The above deflection is quite a significant one to prevent that any large sized components, such as the prisms 7 and 8 and the image microforming device 9, bearing an associated rather voluminous piloting card not shown in FIG. 1, may interfere with the optical path of the light beam along the segment AB and/or the integrating rod 4. This illumination system may also be used in a mirror back projection configuration. i.e. the configuration where the image is projected upwards, since the illumination axis of the lighting lamp 1 is horizontal and substantially perpendicular to the projection axis and therefore, placing the videoprojector 21 upright, so that the projection lens 10 sending the image upwards, not change the position of the lighting lamp 1, which, in the position illustrated in FIG. 1, is in its optimal condition for heat dissipation, warranting a long service life of the videoprojector 21.
However, the illumination system for a videoprojector according to FIG. 1 has the drawback of excessive overall dimensions, particularly for its height, which is due, as mentioned above, to large sized components arranged either above or near to the segment AB of the optical path.
A basic diagram of a second known illumination system of a videoprojector 31 is represented in the configuration of FIG. 2. The optical path, represented by the dotted line extending through the points ABBxe2x80x2CDE, is deflected by three reflecting surfaces, indicated respectively with the reference numbers 6xe2x80x2, 6xe2x80x3, 6xe2x80x2xe2x80x3; the surfaces 6xe2x80x2 and 6xe2x80x3 reflect the optical path downwards, whereas the surface 6xe2x80x3, reflects it upwards. As it can be noticed, the development of the optical path is such that no interference problems exist with the larger components. This allows to have more restricted height dimensions with respect to the solution of FIG. 1. With this system, the light beam is focused on the input of the projection lens 10 instead of the image microforming device 9, according to the known Kohler configuration; the reflecting surface 6xe2x80x2xe2x80x3 will provide a correct angle shot of the optical beam sent to the image microforming device 9, whereby both prisms 7 and 8 of FIG. 1 are not required; the set of the three lenses or relay lens indicated with the reference number 5 provide for the image focusing.
However, the system of FIG. 2 cannot be used in a mirror back projection configuration, this is due to the fact that the lamp axis, coinciding with the segment AB, is horizontal but not substantially perpendicular to the projection axis represented by the segment DE, therefore, should the videoprojector 31 be positioned upright, as requested for a mirror back projection, the lamp axis would be inclined upwards, in which case the lamp cannot ensure optimal heat dissipation and a considerable shorter life would result.
A further drawback for the system represented in FIG. 2 is due to the fact that the image focused on the projection lens input is smaller, since the light rays from the image microforming device 9 are converging rays, so the projection lens 10 cannot be shifted vertically, as a portion of the light beam from the image microforming device 9 would go lost; as a result, the image position cannot be adjusted on the screen, i.e. there is no possibility of performing a so-called vertical offset.
Therefore, common illumination systems have several drawbacks, the most significant ones being their big size, the impossibility of using the videoprojector in all possible configurations (frontal, ceiling projection, back projection) and adjusting the upright image position (so-called offset). In particular, in the mirror back projection widely used in television sets with a screen over 40xe2x80x3, the videoprojector is positioned upright and send the light beam upwards to a mirror, which will reflect it back on the screen, with ensuing cooling problems for the lamp.
It is the aim of the present invention to provide an optic system of illumination for videoprojectors, which can solve the above drawbacks and allows the manufacture of videoprojectors with restricted dimensions and usable in any configuration. In order to achieve such aims, it is the object of the present invention an optic system of illumination incorporating the features of the annexed claims, which form an integral part of the description herein.